Lots of discussion on this.gvl280 said:
Go to this thread:
http://www.clubarmada.com/forums/showthread.php?t=965&highlight=thule
and/or use the search function for "thule" good luck! I have a modified (to fit the stretch between bars) Thule 1800 box with an additional stiffener pop-riveted in one of the channels at the bottom of the box to increase bottom stiffness. Works great.
Post script - field tested to 160kph (in BC)